1. Field of the Invention
This invention relates to hot liquid coating apparatus and more particularly to an improved fluid nozzle assembly for directing a controlled, thin flat stream of a gaseous fluid onto the surface of a running length of a substrate emerging from a liquid coating bath to wipe the coating liquid from the surface of the substrate to produce a controlled, uniform coating. The nozzle assembly is particularly well adapted for hot dip coating metal strip such as the continuous galvanizing of steel wherein the nozzle assembly is located in a very high temperature environment.
2. Description of the Prior Art
The use of a thin, wide jet of fluid to wipe excess coating fluid from a moving substrates has long been known in the paper coating art, and has more recently been widely accepted in the hot dip coating of metal substrates. Since the nozzle structure of the present invention is particularly well adapted for use in the hot dip galvanizing of metal strip, it will be described herein with reference to such a process, it being understood that the nozzle structure may also be used in other coating operations.
In the hot dip galvanizing of strip steel a fluid such as stream, air, or a mixture of steam and air is directed against the strip surface after it emerges from the spelter bath and while the coating material is still in the liquid state. Normally, two nozzles are employed one on each side of the substrate, with the jet streams being directed in substantially opposed relation so that forces applied to the strip by the fluid jets are substantially equalized. The wide, thin nature of the jet streams extending across the full width of the substrate being coated results in the nozzles normally being referred to as air knives, although the fluid impinging on the moving substrate is believed to act more in the nature of a dam preventing excess coating material from being carried through than a knife or blade which strips the liquid from the substrate.
While the use of air knives has largely replaced the older technique of coating rolls in the hot dip coating of metals, it has not always been possible to achieve the desired accuracy in controlling both the thickness and distribution of the coating material due, in part, to the necessity for accurately controlling various parameters, well known in the art, which affect both distribution and coating thickness. This has been made more difficult by the high temperatures encountered by the nozzle structures which of necessity must be placed very close to the hot dip bath where temperatures are very high. Also, the steam or air conventionally used in hot dip galvanizing processes is maintained at a relatively high temperature to avoid excessive cooling of the coating material. The heat to which the nozzle assemblies are subjected in such an operation has frequently resulted in distortions of the structure and produced an uneven distribution of the gaseous medium across the strip width.
It has been proposed, for example in U.S. Pat. No. 4,041,895 to Overton et al. and U.S. Pat. No. 3,917,888 to Beam et al., to provide a nozzle structure having an enlarged internal plenum chamber into which the wiping fluid is discharged, and to provide perforated baffle plates, screens, guide vanes and the like in the enlarged plenum chamber between the fluid inlet and the nozzle outlet in an effort to produce a more uniform pressure, and therefor more uniform fluid flow from the nozzle across the full width of the outlet. However, the use of separate baffle plates, screens, and the like inserted in the interior of the enlarged cavity necessarily results in a complex nozzle structure assembled from a plurality of components of widely differing shapes assembled together with threaded fasteners. Such complex, assembled structure can tend to aggravate the differential expansion and thermal distortion problems, mentioned above.
U.S. Pat. No. 3,141,194 to Jester discloses an air knife structure employed to direct air onto the surface of a film or synthetic resin material on a casting drum. The air knife structure employs a plenum chamber with two sections joined along their length by a relatively wide slot and a second wide slot leads to a converging nozzle structure which reduces turbulence in the air leading to the exit nozzle.
U.S. Pat. No. 4,078,103 to Thornton et al. discloses a metal strip coating apparatus employing an air knife for directing a stream of air onto the surface of the substrate to control thickness and distribution of the molten metal. The nozzle structure employs a pair of opposed inlet chambers each having an elongated outlet slot extending substantially throughout its length for discharging air under pressure into a common plenum chamber. The plenum chamber has a so-called "bow tie" shaped outlet nozzle for directing the air onto the surface of the strip as it emerges from the hot dip bath. Shims between the two nozzle sections are employed to control the outlet nozzle size. The outlet slots from the inlet chambers are in closely spaced, opposed relation so the streams impinge directly on one another, thereby creating substantial flow resistance and turbulence.